Illuminating the dark universe in the multi-messenger era

Abstract

The precision era of multi-messenger astronomy, together with modern astrophysical, cosmological, and gravitational wave observations, increasingly points toward the existence of a ``dark" sector that cannot be explained within the framework of the Standard Model of particle physics and General Relativity. In this review, we explore extensions of standard physics and examine how observational data can be used to probe new particles and interactions. We consider a wide range of scales, from Solar System tests to galactic and cosmological observations, and investigate both conventional dark matter candidates, such as weakly interacting massive particles, and alternative scenarios including ultralight fields and primordial black holes. We discuss constraints derived from compact objects such as neutron stars, black holes, pulsars, and magnetars observations as well as from high-energy astrophysical phenomena. In addition, we analyze extensions of General Relativity involving additional scalar fields and their impact on gravitational wave signals and stochastic backgrounds from primordial black holes. We also study the capture and accumulation of dark matter in compact objects, which can alter properties such as mass, radius, and tidal deformability, and consider scenarios in which dark matter decays into Standard Model particles. While current observations already place significant limits on dark matter and modified-gravity models, upcoming experiments and observatories are expected to further probe or discover such new physics by improving constraints on particle masses and interaction strengths.

Figures (29)

• Figure 1: Left: scaled mass--radius relation of a polytropic star ($\Gamma = 2$) for different chameleon masses. Right: the same relation in GR with $\Gamma = 2.23$, which mimics the $\hat{m} = 1000$ chameleon case. Adapted from Ref. Brax:2017wcj.
• Figure 2: The best fit of the S2 motion with conformal and disformal interactions. The evolution of the numerical simulation is for 60 years. Adapted from Ref. Benisty:2021cmq.
• Figure 3: Iterated retarded positions on the worldline for the two bodies. The disformal coupling always has two legs out of a vertex. The exchange could be a scalar or a graviton and propagates along geodesics between the two bodies. Adapted from Ref. Davis:2019ltc.
• Figure 4: Resummation of the ladder, where $c$ and $d$ stand for conformal and disformal. Adapted from Ref. Davis:2019ltc.
• Figure 5: Number of GW cycles $\mathcal{N}_\text{pert}$ within the frequency band ${f \in [f_1,f_2]}$ during the inspiral of a double NS binary for different values of the disformal coupling scale $\mathcal{M}$. Each type of NS (labeled W, S, or S${}^{\ast}$) are listed in Table \ref{['table:neutron_star_parameters']}. $\mathcal{N}_\text{pert}$ splits into four parts: $\mathcal{N}_\text{pert}^{{(\mathrm{GR})}}$ is the prediction of GR, while $\delta\mathcal{N}_\text{pert}^{}{\small\textsc{[o]}}$, $\delta\mathcal{N}_\text{pert}^{{(\mathrm{C})}}{\small\textsc{[so]}}$, and $\delta\mathcal{N}_\text{pert}^{{(\mathrm{D})}}{\small\textsc{[so]}}$ are the scalar spin-independent, conformal spin-orbit, and disformal spin-orbit contributions. The three values used for the lower frequency bound $f_1$ give the beginning of the sensitivity bands for Advanced LIGO, Cosmic Explorer, and the Einstein Telescope. The upper frequency bound $f_2$ corresponds to the end of the perturbative inspiral phase. For values of $\mathcal{M}$ above a threshold shown in each panel as a vertical grey line, the binary goes directly from its perturbative inspiral phase into the merger phase, implying that ${f_2 = f_\text{contact}}$, with ${ f_\text{contact} \sim 1~\text{kHz} }$. For lower values of $\mathcal{M}$, the binary system goes into a nonperturbative inspiral phase and ${f_2 = f_\text{nonpert}}$, with $f_\text{nonpert}$ given by Eq. \ref{['eq:obsv_f_nonpert']} as noted in the main text. The values of $f_2$ are shown along the top axis, and ${\mathcal{N}_\text{pert} = 0}$ when ${ f_2 \leq f_1}$. In each panel, there is a horizontal grey line giving the threshold for at least one GW cycle. Adapted from Ref. Brax:2021qqo.